1. Field of the Invention
This invention is in the field of frame-sequential color video systems, which are systems for sensing or displaying color video images wherein the system senses or displays images in one of the three primary colors after another. Such systems can be realized using color-selective filters, by which we mean any optical filter which transmits or reflects a certain portion of the visible spectrum and can be switched between any number of predetermined colors. Our invention teaches filters which have the ability to switch discretely between the three primary colors of red, green, and blue by employing the electro-optic properties of the Surface Stabilized Ferroelectric Liquid Crystal (SSFLC) device. Such color-selective filters employing SSFLC devices have the required response time for use in frame-sequential color video, where an arbitrary color is "mixed" from appropriate intensities of each of the three primary colors; each color being presented as a time-sequential image frame. The frame-sequential color scheme can equally-well be used for video cameras or video displays.
2. Description of the Prior Art
Color-selective filters useful for frame-sequential color video systems are known in the prior art. Such devices have relied usually on a mechanical, electro-mechanical, or electro-optical tuning means. The simplest example of the former comprises a rotating wheel with various color filters which can be passed in front of an aperture. A Lyot filter, described by John W. Evans, "The Birefringent Filter," J. Opt. Soc. Am.39, 229 (1949), which is well known in the literature, has been demonstrated by Billings, (Bruce H. Billings, "A Tunable Narrow-Band Optical Filter," J. Opt. Soc. Am. 37, 738 (1947)), which can be mechanically tuned through a color range by altering the thickness of stretchable polymer layers which serve to modulate the optical thickness of the filter stages. More recently, Title et al., (A. M. Title and W. J. Rosenberg, "Tunable birefringent networks," Active Optical Devices, Proc. SPIE 202, 48 (1979)), have developed an electro-mechanical method of continuously tuning the Lyot filter by means of rotating half wave-plates inside each filter stage. They have also applied the same means, later refined by Buhrer et al., (C. F. Buhrer, "Four waveplate dual tuner for birefringent filters and multiplexers," Appl. Opt., 26, 3628 (1987)), for color tuning of a Solc filter, (I. Solc, "Birefringent Chain Filters," J. Opt. Soc. Am., 55, 621 (1965); John W. Evans, "Solc Birefringence Filter," J. Opt. Soc. Am. 48, 142 (1958)), also well known in the literature. Such mechanical or electro-mechanical tuning methods are unsuitable for display applications because of their slow response, even when driven electrically, and because of the bulkiness of a device which relies on the above tuning means.
Billings (Billings, op tit.) has suggested a means of electro-optically tuning a Lyot filter by employing Kerr cells in each stage of the filter. An electric field applied to the Kerr cells causes an induced birefringence which alters the optical thickness of the filter stages and the resultant peak transmission is shifted in wavelength. This device is fast enough to be suitable for display applications, even for frame-sequential color displays which require at least a 10 ms response time per frame. Billings proposed a design giving the three primary colors of red, green, and blue with the above system. A Solc type filter has been demonstrated by Harris et al. (S. E. Harris, and R. W. Wallace, "Acousto-Optic Tunable Filter," J. Opt. Soc. Am., 59, 744 (1969)), which is tuned from 400 nm to 700 nm by varying the frequency of an acoustic wave propagating collinearly with the optical signal along a crystal of LiNbO.sub.3. The passband of this filter is extremely narrow, being about 0.04 nm, and the angular aperture is also small, being about 15.degree.. The resulting low light levels and narrow viewing cone of such a filter make it unsuitable for display applications. A filter similar to the above, but in which a Solc type geometry is induced by means of an array of DC voltage levels along the length of a LiTaO.sub.3 crystal, was demonstrated by Lotspeich et al. (J. F. Lotspeich, R. R. Stephens, and D. M. Henderson, "Electro-Optic Tunable Filter," Opt. Eng., 20, 830 (1981)). Color tunability across the visible spectrum is achieved by varying the voltage levels.
Color-selective filters employing active liquid crystal elements (most of them nematic liquid crystals) are well known in the prior art. Such filters usually rely on the selective absorption or the selective interference of light as the basis for generating color. One such filter of the former type, disclosed in U.S. Pat. No. 4,770,500, employs liquid-crystal quarter-wave cells and a passive quarter wave-plate to rotate by 90.degree. orthogonally polarized green and red light from a pleochroic polarizer, and to transmit one of these colors, or a combination color through a linear polarizer. As this device is restricted to only two primary colors, it is clearly unsuitable for full color display applications. Another such device, called the "field sequential color converter," (M. G. Clark, and I. A. Shanks, Proc. SID Symp. Dig. 172 (1982)) again employs pleochroic polarizers to orthogonally polarize green and red light from a CRT (cathode-ray tube) and uses a twisted nematic (TN) liquid crystal cell to transmit either color through a linear polarizer. The above filter can also be extended to a three color system as is taught in U.S. Pat. No. 4,416,514. A variation on the above, disclosed in U.S. Pat. No. 4,582,396, uses a nematic liquid crystal "Pi-cell" in place of the TN cell and has the ability to switch between colors at a significantly faster rate (5 ms). This latter variation if extended to a full color system, could be employed in frame-sequential color displays, as described by Haven, (Thomas J. Haven, "Reinventing the color wheel," Information Display, 7, 11 (1991)). U.S. Pat. No. 4,867,536 teaches a color-selectable liquid-crystal display system which employs TN cells and pleochroic polarizers to transmit one of a selection of colors. The system operates much on the same principles as the field-sequential color converter with the addition of employing a display cell with patterned electrodes to enable colors to appear on a black or white background. Similar devices, again employing TN cells and pleochroic polarizers, are disclosed in U.S. Pat. Nos. 4,025,164, 4,497,543, and 4,416,514. A full-color filter employing three zero-twist nematic (ZTN) guest-host cells and a neutral polarizer, has been developed by Uchida et al., (T. Uchida, H. Seiki, C. Shishido, and M. Wada, Proc. SID 22, 41 (1981)). The device operates on the principal of subtractive color mixing, and each of the three ZTN cells is doped with a pleochroic dye suitable for this end.
The prior art shows many examples of using active liquid crystal elements in color-selective interference filters. Scheffer (T. J. Scheffer, "New multicolor liquid crystal displays that use a twisted nematic electro-optical cell," J. Appl. Phys., 44, 4799 (1973)), has developed a color-selective filter which consists of a twisted nematic electro-optical cell and birefringent plate, placed between parallel polarizers. The liquid crystal cell, upon application of an electric field, serves to modulate the spectral transmission of the filter. A two-color filter is obtained with the arrangement described, while four colors can be obtained if two such "filter stages" are cascaded together. Three of the four above colors demonstrated by Scheffer were the primaries of red, green, and blue. A color-selective filter with zero-twist nematic cells as voltage-controllable retarders in a Lyot filter arrangement is disclosed in U.S. Pat. No. 4,394,069. The device is similar to the one developed by Billings in that an electrically induced birefringence change in the ZTN cells changes the design wavelength of the filter thus allowing for continuous tuning between colors. The use of ZTN cells however, renders this filter slower than the one demonstrated by Billings.
Nematic liquid crystals have several advantages for use in color-selective filters, namely, they exhibit large modulations for relatively small changes in applied voltage and their operation consumes very little electrical power. However, nematic liquid crystals respond to changes in the magnitude and not the polarity of an applied electric field. Therefore although they can be switched rapidly in one direction by an applied field, they are switched in the opposite direction by removing the field and their response time, governed by relatively slow elastic restoring forces, is limited to about 20 ms. Such a response time is much too slow for frame-sequential color displays where the overall picture color is "mixed" in time-sequential frames of the three primary colors. Current nematic liquid crystal displays "mix" the picture color spatially from a miniature triad of three primary color filters. This spatial arrangement suffers from poorer picture resolution when compared to the temporal one. For frame-sequential display applications, a liquid crystal is needed which responds in less than the 10 ms required for a "flicker free" display. Ferroelectric liquid crystals (FLC's), responding to the polarity and magnitude of an applied electric field, switch in the 100 us regime which is easily fast enough for the above requirement. A frame-sequential color display system was pointed out by White (J. C. White, "Color LCD TV," Phys. Technol., 19, 91 (1988)), where he suggested the use of fluorescent tubes with rapidly decaying phosphors as a means for color-sequential backlighting of an FLC monochrome display.
As is the case with TN cells, surface-stabilized ferroelectric liquid crystal (SSFLC) devices, which have been well documented in the literature, (K. Skarp, M. A. Handschy, "Ferroelectric liquid crystal material properties and applications," Mol. Cryst. Liq. Cryst., 165, 439 (1988); N. A. Clark, M. A. Handschy, and S. T. Lagerwall, Mol. Cryst. Liq. Cryst., 94, 213 (1983)) can also be used in conjunction with pleochroic dyes or birefringent elements, to generate color. A liquid crystal device, employing a ferroelectric liquid crystal cell and a birefringent film, placed between parallel or crossed polarizers, is disclosed in U.S. Pat. No. 4,711,530. This device employs the selective interference of light to improve the display quality of the liquid crystal cell by suppressing a range of transmitted colors. A method of fabricating a wavelength-selective filter by incorporating SSFLC devices in a Lyot filter is taught by Masterson et al. (H. J. Masterson, G. D. Sharp, and K. M. Johnson, "A Ferroelectric Liquid Crystal Tunable Filter," Opt. Lett., 14, 1249 (1989). The ability to operate the SSFLC devices as switchable waveplates is employed to modulate the retardation of each stage of the filter and so alter the range of transmitted wavelengths. This paper describes a canonical Lyot filter, which is made up of several stages of retarders between crossed polarizers, with the optical retardance of each stage being twice that of the previous stage. This paper further teaches that the nth stage of the Lyot filter requires 2.sup.n-1 FLC devices. Hence, the three-stage Lyot filter demonstrated in this paper required seven FLC devices. The filter was used to demonstrate switching between two different spectral bands. This type of filter, when implemented with two or more stages, could give adequate color saturation for frame-sequential video applications, but the number of FLC devices required, produces a complex and expensive filter that is difficult to manufacture.
A continuously tunable filter employing a smectic A* liquid crystal electroclinic cell, (G. Andersson, I. Dahl, P. Keller, W. Kuczynski, S. T. Lagerwall, K. Skarp, and B Stebler, "Submicrosecond electro-optic switching in the liquid crystal smectic A* phase: The soft mode ferroelectric effect," Appl. Phys. Lett., 51, 640 (1987)) has been demonstrated by Sharp et al. (G. D. Sharp, K. M. Johnson, and D. Doroski, "Continuously tunable smectic A* liquid crystal color filter," Opt. Lett., 15, 523 (1990)). The filter employs an electroclinic cell in conjunction with a birefringent element and quarter wave-plate, placed between parallel polarizers, to continuously tune the transmission across a portion of the visible spectrum. Although the filter can be tuned rapidly in &lt;100 ns, it is only operable in a very narrow, controlled temperature range. Furthermore, it does not have a wide enough tuning range to include the three additive primary colors. It is therefore unsuitable for display applications.